This invention relates to a method of making a cell component containing a polymer, to a cell component so made, and to an electrochemical cell incorporating this cell component.
For many years it has been known to make rechargeable cells with lithium metal anodes, and cathodes of a material into which lithium ions can be intercalated or inserted. Such cells may use a separator such as filter paper or polypropylene saturated with, as electrolyte, a solution of a lithium salt (e.g. lithium perchlorate) in an organic liquid such as propylene carbonate. Alternatively they may use a solid-state ion-conducting polymer, for example a complex of a lithium salt with poly(ethylene oxide). A wide variety of intercalation or insertion materials are known as cathode materials, such as TiS2, V6O13 and LixCoO2 where x is less than 1; and these materials are often mixed with solid electrolyte material to form a composite cathode. To avoid the problems arising from dendrite growth at the anode, it has been proposed to use an intercalation material such as graphite as the anode material too, and this also may be mixed with solid electrolyte material to form a composite anode. Similar cells can be made in which sodium takes the place of lithium.
An alternative type of polymer electrolyte has recently been proposed by Gozdz et al (U.S. Pat. No. 5 296 318), which comprises a copolymer of 75 to 92% vinylidene fluoride and 8 to 25% hexafluoropropylene, blended with a lithium salt and a compatible solvent such as ethylene carbonate/propylene carbonate mixture and cast from solution in a low boiling-point solvent such as tetrahydrofuran. This is said to provide a stable film with conductivity in the range 10xe2x88x924 to 10xe2x88x923 S cmxe2x88x921. Patent application GB 2 309 701 A describes an electrolyte in which the PVdF/HFP copolymer is replaced by a polymeric chain consisting primarily of vinylidene fluoride onto which is grafted a mono-unsaturated carboxylic, sulphonic or phosphonic acid, ester or amide; as in Gozdz the electrolyte also comprises a salt and an organic solvent such as ethylene carbonate and/or propylene carbonate, and it is cast from solution in a low boiling point solvent such as dimethylacetamide or tetrahydrofuran. Although this process provides good quality electrolytes, it is inconvenient to have to evaporate the large quantities of low boiling point solvent involved.
According to the present invention there is provided a method of making a composite electrode for a cell, the method comprising mixing a polymer consisting primarily of vinylidene fluoride with an insertion material in particulate form and an organic plasticiser, and extruding the mixture, the organic plasticiser being mixed with the polymer prior to completion of the extrusion, and the extrusion being performed at a temperature above the melting point of the polymer so as to melt cast a non-porous film or sheet of the composite electrode.
The mixture to be extruded may also include an appropriate salt. The cell component may be a composite cathode or a composite anode, by incorporating an appropriate insertion material.
If the cell is a lithium cell (or a lithium ion cell) then the salt will be a lithium salt, such as lithium perchlorate LiClO4. Other suitable salts are LiAsF6, LiPF6, LiBF4, LiN(SO2CF3)2, or LiCF3SO3; if the salt is to be extruded then it must be stable at the extrusion temperature. A variety of compatible plasticisers may be used, in particular ethylene carbonate or propylene carbonate; alternative plasticisers are diethoxyethane or diethyl carbonate. Plasticisers such as tetraethylene glycol dimethyl ether (tetraglyme), or N-methyl-pyrrolidone (1-methyl-2-pyrrolidone) may also be provided, to ensure that other plasticisers do not crystallise at the desired operating temperature and so to ensure adequate electrical conductivity. Other plasticisers which may be used are phthalates such as dibutylphthalate, esters of aliphatic dibasic acids such as dioctyladipate or dibutylsebacate, alkyl phosphates such as tributylphosphate, and polymeric polyesters of adipic acid such as poly (1, 3 butylene glycol/1, 2 propylene glycol/adipic acid ester). It will be appreciated that the plasticiser must have a boiling point above the extrusion temperature to ensure that at least the bulk of the plasticiser does not evaporate during extrusion.
The polymer comprises a polymeric chain consisting primarily of vinylidene fluoride and may be a homopolymer, i.e. polyvinylidene fluoride (PVdF), or may be a copolymer or terpolymer of vinylidene fluoride (VdF) and other monomers such as hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), or tetrafluoroethylene (TFE), and in these cases the proportion of VdF is preferably no less than 70% by weight. The polymer may have further monomers grafted onto such a polymeric chain, in particular a mono-unsaturated carboxylic, sulphonic or phosphonic acid, ester or amide may be grafted onto the chain. Such a grafted polymer may have improved adhesion to metal components such as current collectors in a cell.
The monomers to be grafted must have only one double-bond in the carbon chain Rxe2x80x94, and one or more carboxyl groups xe2x80x94COOH, sulphonic acid groups xe2x80x94SO2OH, phosphonic acid groups xe2x80x94PO(OH)2, ester groups xe2x80x94COORxe2x80x2, or amide groups xe2x80x94CONH2. Generally smaller monomers, with less than five carbon atoms in the carbon chain Rxe2x80x94, are preferable. For example acrylic acid; crotonic acid, vinylacetic acid, methylacrylic acid (isomers of butenoic acid); isomers of pentenoic acid such as allylacetic acid, or tiglic acid; or, as examples of monomers with more than one acid group, itaconic acid or maleic acid. The corresponding amides such as acrylamide may also be used. In an ester, the group Rxe2x80x2 might be methyl, ethyl, hydroxyethyl, or butyl; for example esters such as methyl acrylate or hydroxyethyl methacrylate may be used. Vinyl or styrene sulphonic acid may be used. The most preferred monomers to be grafted are acrylic acid or methacrylic acid. The grafting may be achieved by an irradiation process. For example the polymer chain substrate and the graft monomer material together may be subjected to continuous or intermittent irradiation, or more preferably the substrate may be pre-irradiated before it is brought into contact with the graft monomer material. The radiation may for example be an electron beam, or X-rays or xcex3-rays. The irradiation activates the substrate, apparently by generating free radicals.
The degree of grafting is determined by several factors, the most important being the length of time that the activated substrate is in contact with the graft monomer material, the extent of pre-activation of the substrate by the irradiation, the extent to which the graft monomer material can penetrate the substrate, and the temperature of the substrate and monomer material when in contact. If the graft monomer material is an acid, the degree of grafting can be monitored by sampling the solution containing the monomer, and titrating against a base to find the concentration of the acidic monomer remaining. The degree of grafting in the resulting composition is desirably between 2 and 20% of the final weight, more preferably between 3 and 12%, for example 5% or 10%.
In a second aspect the present invention provides a composite electrode in the form of a sheet or film of non-porous polymer-containing material made by extrusion as specified above; and in a third aspect it provides an electric cell incorporating at least one such composite electrode.
Thus the cell may incorporate a composite cathode made by extrusion, and/or a composite anode so made. The referred cell is a rechargeable lithium swing cell (that is, without lithium metal) with a composite anode and a composite cathode each comprising polymer mixed with appropriate insertion materials, each being made by extrusion, and separated by an electrolyte layer which may also be made by extrusion. The composite anode and the composite cathode must each be in contact with a suitable current collector such as copper or aluminium foil or mesh. In one method a composite electrode is extruded in tubular form, a strip of current collector foil is then inserted into the tube, and the tube is then squashed flat so the foil is sandwiched between sheets of composite electrode material. This last step may be performed using heated rollers to ensure the foil is bonded to the composite electrode material. In an alternative method a composite anode, a polymer electrolyte, and a composite cathode are all extruded, are then sandwiched between metal foils, and are bonded together by passing between heated rollers.